HUMAN EYE: Basic structural outline and components (accessory structure of eye).

Sight or vision is extremely important to human survival. More than half the sensory receptors in the human body are located in the eyes, and a large part of the cerebral cortex is devoted to processing visual information. In this section of the chapter, we examine electromagnetic radiation, the accessory structures of the eye, the eyeball itself, the formation of visual images, the physiology of vision, and the visual pathway from the eye to the brain.

Accessory structure of eye

The accessory structures of the eye include the eyelids, eyelashes, eyebrows, the lacrimal (tearing) apparatus, and extrinsic eye muscles.

Eyelids

The upper and lower eyelids, or palpebrae (singular is palpebra), shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs The upper eyelid is more movable than the lower and contains in its superior region the levator palpebrae superioris muscle. Sometimes a person may experience an annoying twitch in an eyelid, an involuntary quivering similar to muscle twitches in the hand, forearm, leg, or foot. Twitches are almost always harmless and usually last for only a few seconds. They are often associated with stress and fatigue. The space between the upper and lower eyelids that exposes the eyeball is the palpebral ﬁssure. Its angles are known as the lateral commissure, which is narrower and closer to the temporal bone, and the medial commissure, which is broader and nearer the nasal bone. In the medial commissure is a small, reddish elevation, the lacrimal caruncle, which contains sebaceous (oil) glands and sudoriferous (sweat) glands. The whitish material that sometimes collects in the medial commissure comes from these glands. From superﬁcial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, and ﬁbers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva. The tarsal plate is a thick fold of connective tissue that gives form and support to the eyelids. Embedded in each tarsal plate is a row of elongated modiﬁed sebaceous glands, known as tarsal or Meibomian glands, that secrete a ﬂuid that helps keep the eyelids from adhering to each other. Infection of the tarsal glands produces a tumor or cyst on the eyelid called a chalazion (small bump). The conjunctiva is a thin, protective mucous membrane composed of non-keratinized stratiﬁed columnar epithelium with numerous goblet cells that is supported by areolar connective tissue. The palpebral conjunctiva lines the inner aspect of the eyelids, and the bulbar conjunctiva passes from the eyelids onto the surface of the eyeball, where it covers the sclera (the “white” of the eye) but not the cornea, which is a transparent region that forms the outer anterior surface of the eyeball. Over the sclera, the conjunctiva is vascular. Both the sclera and the cornea will be discussed in more detail shortly. Dilation and congestion of the blood vessels of the bulbar conjunctiva due to local irritation or infection are the cause of bloodshot eyes.

Eyelashes and Eyebrows

The eyelashes, which project from the border of each eyelid, and the eyebrows, which arch transversely above the upper eyelids, help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun. Sebaceous glands at the base of the hair follicles of the eyelashes, called sebaceous ciliary glands, release a lubricating ﬂuid into the follicles. Infection of these glands is called a sty.

The Lacrimal Apparatus

The lacrimal apparatus (lacrim - tears) is a group of structures that produces and drains lacrimal ﬂuid or tears. The lacrimal glands, each about the size and shape of an almond, secrete lacrimal ﬂuid, which drains into 6 to 12 excretory lacrimal ducts that empty tears onto the surface of the conjunctiva of the upper lid. From here the tears pass medially over the anterior surface of the eyeball to enter two small openings called lacrimal puncta (singular is punctum). Tears then pass into two ducts, the lacrimal canals, which lead into the lacrimal sac and then into the nasolacrimal duct.

This duct carries the lacrimal ﬂuid into the nasal cavity just inferior to the inferior nasal concha. An infection of the lacrimal sacs is called dacryocystitis (dacryo - lacrimal sac; -itis - inﬂammation of). It is usually caused by a bacterial infection and results in blockage of the nasolacrimal ducts. The lacrimal glands are supplied by parasympathetic ﬁbers of the facial (VII) nerves. The lacrimal ﬂuid produced by these glands is a watery solution containing salts, some mucus, and lysozyme, a protective bactericidal enzyme. The ﬂuid protects, cleans, lubricates, and moistens the eyeball. After being secreted from the lacrimal gland, lacrimal ﬂuid is spread medially over the surface of the eyeball by the blinking of the eyelids. Each gland produces about 1 mL of lacrimal ﬂuid per day. Normally, tears are cleared away as fast as they are produced, either by evaporation or by passing into the lacrimal canals and then into the nasal cavity. If an irritating substance makes contact with the conjunctiva, however, the lacrimal glands are stimulated to over secrete, and tears accumulate (watery eyes). Lacrimation is a protective mechanism, as the tears dilute and wash away the irritating substance. Watery eyes also occur when an inﬂammation of the nasal mucosa, such as occurs with a cold, obstructs the nasolacrimal ducts and blocks drainage of tears. Only humans express emotions, both happiness and sadness, by crying. In response to parasympathetic stimulation, the lacrimal glands produce excessive lacrimal ﬂuid that may spill over the edges of the eyelids and even ﬁll the nasal cavity with ﬂuid. This is how crying produces a runny nose.

Extrinsic Eye Muscles

The extrinsic eye muscles extend from the walls of the bony orbit to the sclera (white) of the eye and are surrounded in the orbit by a signiﬁcant quantity of periorbital fat. These muscles are capable of moving the eye in almost any direction. Six extrinsic eye muscles move each eye: the superior rectus, inferior rectus, lateral rectus, medial rectus, and superior oblique and inferior oblique. They are supplied by cranial nerves III, IV, or VI. In general, the motor units in these muscles are small. Some motor neurons serve only two or three muscle ﬁbers—fewer than in any other part of the body except the larynx (voice box). Such small motor units permit smooth, precise, and rapid movement of the eyes. The extrinsic eye muscles move the eyeball laterally, medially, superiorly, and inferiorly. For example, looking to the right requires simultaneous contraction of the right lateral rectus and left medial rectus muscles of the eyeball and relaxation of the left lateral rectus and right medial rectus of the eyeball. The oblique muscles preserve rotational stability of the eyeball. Neural circuits in the brain stem and cerebellum coordinate and synchronize the movements of the eyes.